Modems are widely used in telecommunications for communication between systems, e.g. for satellite communications and for telecommunications generally.
In the past, all of the main indications involved in transmission and reception have been carried out using analog or a combination of digital and analog techniques, particularly with respect to carrier-recovery functions.
With the development of domestic satellite telecommunication systems, especially for teledetection, low-bitrate commercial services, telemonitoring, telecontrol and multichannel signalling, for which spread-spectrum transmission is particularly well suited when the satellite access protocol is of the code-division multiple access (CDMA) type, spread-spectrum modems have had to be developed for civilian purposes. In developing such modems for such telecommunication purposes, however, the modems have retained their essentially analog topology.
In particular, the main code-carrier recovery functions were achieved at intermediate frequency levels utilizing analog devices. Both the main code recovery and the carrier-recovery operation were performed at such intermediate frequencies with the analog devices which had been found to be suitable for such modems prior to their increased use in telecommunications for civil purposes.
The analog approach for main code recovery and for carrier recovery, however, requires very complex circuitry which could not be tightly packaged and also had high power consumption and cost. The flexibility of the system was limited and generally it was difficult to interface such modems with high intelligence systems or high echelon processes or use such modems to interface with higher echelon processes and more intelligent telecommunication devices and systems.